A key point regarding your first item is that it isn't particularly the language that is easy to pick up - it is that its original "native" environment made it very easy to access and play with. People often start with a bit of JS to tart up their web page, then move to making it even more interactive, and once they start wanting server activity there is now the option of not needing to learn another language for that. The single language server and client side is quite a draw, and with some frameworks where your code is running can actually be somewhat obscured so you don't need to care.